1. Field of the Invention
The present invention relates to a method of operating a phosphoric acid fuel cell (PAFC) including an assembly which comprises an anode, a cathode, and an electrolyte layer impregnated with a phosphoric acid and interposed between the anode and the cathode.
2. Description of the Related Art
Each unit of the phosphoric acid fuel cell has an electrolyte electrode assembly comprising an anode, a cathode, and an electrolyte layer impregnated with phosphoric acid and interposed between the anode and the cathode. The matrix which is impregnated with phosphoric acid is generally made of porous body of silicon carbide. However, it has been proposed to use a basic polymeric membrane of polybenzimidazole or the like (see U.S. Pat. No. 5,525,436).
Usually, the phosphoric acid fuel cell includes a predetermined number of unit cells which are stacked together and electrically connected in series to form a stack body.
For operating the phosphoric acid fuel cell, the anode is supplied with a fuel gas (hydrogen-containing gas), and the cathode is supplied with an oxygen-containing gas. Hydrogen in the hydrogen-containing gas which is supplied to the anode is ionized by the anode according to the following formula (A), generating hydrogen ions and electrons:2H2→4H++4e  (A)
The generated hydrogen ions move through the electrolyte layer to the cathode. The electrons are transmitted to an external circuit which is electrically connected to the anode and the cathode. After the electrons are used as a DC electric energy for energizing the external circuit, they reach the cathode.
The hydrogen ions that have moved to the cathode and the electrodes that have reached the cathode via the external circuit react with oxygen contained in the oxygen-containing gas supplied to the cathode according to the formula (B):O2+4H++4e→2H2O  (B)
The electrochemical reaction is an exothermic reaction. Therefore, heat is generated in operating the fuel cell.
When the phosphoric acid fuel cell is operated at a temperature lower than 100° C. which is the boiling point of water, the ratio of H2O generated as a liquid (water) increases, and the generated water remains stagnant in the phosphoric acid fuel cell. Then, the phosphoric acid in the electrolyte layer is dissolved into the generated water, and discharged together with the generated water when the water is finally discharged out of the fuel cell. As a result, the concentration of the phosphoric acid in the electrolyte layer is lowered, degrading the cell characteristics of the phosphoric acid fuel cell.
The phosphoric acid fuel cell is usually operated in a temperature range from 140° C. to 190° C. in order to avoid the above drawback. Specifically, the phosphoric acid fuel cell is heated to a temperature above 100° C. by a heating means such as a heater. Thereafter, the fuel gas and the oxygen-containing gas are supplied to react as described above, and the fuel cell is further heated up to the temperature range from 140° C. to 190° C. with the heat generated by the reaction.
However, the above heating process is disadvantageous because it takes a long period of time until the phosphoric acid fuel cell reaches the operating temperature. The phosphoric acid fuel cell thus suffers a practical problem in that it cannot generate electric energy immediately when desired because it takes a long period time until the fuel cell becomes ready to operate stably in a steady state.
One solution would be to use a heater having a large heat-generating capability. However, such a heater would be large in size, make the overall fuel cell system also large in size, and greatly lower the energy efficiency per volume and weight.
Japanese laid-open patent publication No. 63-225477 and U.S. Pat. No. 6,103,410 disclose a process of supplying a fuel gas and an oxygen-containing gas simultaneously to a cathode of a fuel cell, causing a reaction represented by the formula (C) shown below, and heating the phosphoric acid fuel cell with the heat generated in the reaction:2H2+O2→2H2O  (C)
Even according to the above disclosed process, liquid water is generated when the temperature of the phosphoric acid fuel cell is lower than 100° C. Therefore, the phosphoric acid in the electrolyte layer is dissolved into the generated water, and discharged together with the generated water out of the fuel cell. Consequently, it is impossible to avoid the shortcoming that the cell characteristics of the phosphoric acid fuel cell are lowered simply by the reaction of hydrogen and oxygen.